1. Field of the Invention
The present invention relates to an antenna structure used for radio communication and a communication apparatus including the same.
2. Description of the Related Art
Various types of antenna structures to be provided in radio communication apparatuses have been proposed. For example, in the antenna structure disclosed in Japanese Unexamined Patent Application Publication No. 11-8508 (reference 1), a reinforcing portion 31 made of resin is integrally formed in an antenna portion 30 including a plate, as shown in FIG. 17B. The antenna portion 30 is attached to a printed wiring board 32, as shown in FIG. 17A.
Also, Japanese Unexamined Patent Application Publication No. 10-32409 (reference 2) discloses an antenna structure shown in FIG. 18. In this antenna structure, a plate antenna 35 is integrated into a casing 36. The casing 36 encases components mounted on a printed board 37 (the components are mounted on the back surface of the printed board 37, and thus are not shown in FIG. 18).
Further, the antenna structure disclosed in Japanese Unexamined Patent Application Publication No. 2002-124811 (reference 3) is shown in the cross-sectional view in FIG. 19. In this structure, an antenna 41 is located in a space 45 defined by one end of a circuit board 42, a front cover 43, and a back cover 44, along the internal surface of the back cover 44. Further, an antenna-grounding surface 46 is located along the internal surface of the front cover 43, which faces the antenna 41 with a space therebetween. The antenna 41 and the antenna-grounding surface 46 are connected to the circuit board 42 via conductors 48. Reference numeral 47 denotes a speaker, which is a component of a communication apparatus.
In portable communication apparatuses, the size and thickness are required to be reduced. In order to satisfy this requirement, the size and thickness of antennas used for the apparatuses should be reduced. Accordingly, in the antenna structures of the references 1 to 3, the profile of the antennas 30, 35, and 41 relative to the circuit boards 32, 37, and 42, respectively, should be lowered so as to reduce the thickness of the antennas. However, the profile of the antennas 30, 35, and 41 has an effect on a bandwidth of radio waves for communication of the antennas 30, 35, and 41. Therefore, by lowering the profile of the antennas 30, 35, and 41, the bandwidth of the antennas 30, 35, and 41 becomes narrow.
Further, if the area of each of the antennas 30, 35, and 41 is reduced in order to miniaturize the antenna structure, the antenna gain is disadvantageously deteriorated.
Also, if the size and thickness of the antennas 30, 35, and 41 are simply reduced, the resonance frequency of the antennas 30, 35, and 41 is changed from a set frequency. Therefore, when the size and thickness of the antenna structure are reduced, the resonance frequency of the antennas 30, 35, and 41 must be matched to the set frequency. In that case, however, if an object serving as a ground, such as a shield case, approaches the antenna 30, 35, or 41, the antenna characteristic is significantly deteriorated.